1. Field of the Invention
The present invention relates to X-ray detectors. More particularly, it relates to Thin Film Transistor (TFT) array substrates for use in X-ray detectors.
2. Description of Related Art
A widely used method of medical diagnosis is the X-ray film. As such films produce photographic images, time consuming film-processing procedures are required to obtain the results. However, digital X-ray sensing devices (referred to hereinafter as X-ray detectors) that employing thin film transistors have been developed. Such X-ray sensing devices have the advantage of providing real time diagnosis.
FIG. 1 is a cross-sectional view illustrating one pixel of an array substrate of a related art X-ray detector. That X-ray sensing device includes a Thin Film Transistor (TFT) xe2x80x9cTxe2x80x9d on a substrate 1, a photoconductive film 2, and various conductive elements that are described subsequently. Also included, but not shown in FIG. 1, are a scanning integrated circuit and a data integrated circuit.
Still referring to FIG. 1, the photoconductive film 2 produces electron-hole pairs 6 in proportion to the strength of incident radiation, such as X-rays. Thus, the photoconductive film 2 acts as a photoelectric transducer that converts incident X-rays into electron-hole pairs 6. An external voltage Ev is applied across a conductive electrode 7 and a pixel electrode 62. That voltage causes the electron-hole pairs 6 in the photoconductive film 2 to separate such that X-ray induced electrical charges accumulate on the pixel electrode 62. Those electrical charges are applied to a second capacitor electrode 60, and are consequently stored in a storage capacitor xe2x80x9cSxe2x80x9d formed by the second capacitor electrode 60 and a first capacitor electrode 58 that is formed over a ground line 42. The pixel electrode 62, the first capacitor electrode 58 and the second capacitor electrode 60 are beneficially comprised of a transparent conductive material such as Indium-Tin-Oxide (ITO). Furthermore, an insulating dielectric layer 15 is interposed between the first capacitor electrode 58 and the second electrode 60. That dielectric layer is beneficially comprised of Silicon Nitride (SiNx).
Still referring to FIG. 1, the TFT xe2x80x9cTxe2x80x9d connects to the storage capacitor xe2x80x9cSxe2x80x9d such that electrical charges accumulated on the storage capacitor xe2x80x9cSxe2x80x9d can flow through the TFT xe2x80x9cTxe2x80x9d and into the data integrated circuit (not shown) when the TFT xe2x80x9cTxe2x80x9d is turned ON by the scanning integrated circuit (not shown).
FIG. 2 is a plan view illustrating several pixels of an array substrate for an X-ray detector according to the conventional art. Gate lines 50 are arranged in a transverse direction and data lines 53 are arranged in a longitudinal direction. Gate pads 87 are formed at each end of each gate line 50. Those gate pads 87 are associated with gate pad contact holes 96. The gate pads 87 also connect to a gate shorting bar (not shown) that makes the gate pads have equipotentials. The gate pads 87 are classified into even number gate pads and odd number gate pads when performing short/open-circuit testing. An etch stopper 59 is formed over the gate pads 87.
A TFT xe2x80x9cTxe2x80x9d is formed near each crossing of the gate and data lines 50 and 53 (for simplicity only one TFT xe2x80x9cTxe2x80x9d is shown in detail in FIG. 2). Each TFT acts as a switching element. A ground line 42 is arranged perpendicular to the gate lines 50. That ground line crosses a storage capacitor region xe2x80x9cSxe2x80x9d. The ground line 42 acts as a common line for neighboring pixels.
A first capacitor electrode 58 and a second capacitor electrode 60 of a storage capacitor xe2x80x9cSxe2x80x9d are located in each pixel area, with the pixel areas being the regions between the gate lines and the data lines. Additionally, as shown in FIG. 1, but not shown in FIG. 2, a dielectric layer 15 of Silicon Nitride (SiNx) is interposed between first capacitor electrodes 58 and the second capacitor electrodes 60. Pixel electrodes 62 that extend over the TFTs xe2x80x9cTxe2x80x9d are then located in the pixel areas. Although not shown in FIG. 2, but as shown in FIG. 1, in order to store the holes which are generated in the photoconductive film 2, each pixel electrode 62 electrically connects to the second capacitor electrode 60 of that pixel. Furthermore, each pixel electrode 62 is electrically connected to a drain electrode 33 of that pixel""s TFT xe2x80x9cTxe2x80x9d via a drain contact hole 85.
The fabrication steps of the array substrate illustrated in FIG. 2 will be explained with reference to FIGS. 3A to 3E, which are cross-sectional views taken along lines Ixe2x80x94I, IIxe2x80x94II and IIIxe2x80x94III.
Referring to FIG. 3A, a first metal layer is formed on a substrate 71 by depositing a metallic material such as Aluminum (Al), Al-alloy, Molybdenum (Mo), Tantalum (Ta), Tungsten (W), Niobium (Nb) or Antimony (Sb). A gate line (not shown), a gate electrode 73 that extends from the gate line, and gate pads 87 on each end of the gate line are then formed by patterning the first metal layer. Simultaneously formed are a shorting bar (not shown) and a shorting bar connector (also not shown) that connects the gate pads to the shorting bar. Then, a first insulation layer 75 is deposited over the substrate 71 and over the first patterned metal layer. The first insulation layer 75 can be comprised of an inorganic substance, such as Silicon Nitride (SiNx) or Silicon Oxide (SiOx), or of an organic substance such as BCB (Benzocyclobutene) or an acryl. Silicon Nitride (SiNx) is assumed to be employed hereinafter.
As shown in FIG. 3B, a pure amorphous silicon (a-Si:H) layer and a doped amorphous silicon (n+ a-Si:H) layer are sequentially formed over the first insulation layer 75. Those silicon layers are then patterned to form an active layer 86 and an ohmic contact layer 91. CVD (Chemical Vapor Deposition) or the Ion Injection Method is beneficially used to form the doped amorphous silicon layer.
Referring now to FIG. 3C, a source electrode 32, a drain electrode 33, and a ground line 42 are then formed. First, a second conductive metal layer is deposited. The second conductive metal layer is then patterned to form the source electrode 32, which extends from the data line (reference element 53 of FIG. 2) over the gate electrode 73; the drain electrode 33, which is spaced apart from the source electrode 32 and over the gate electrode 73; and the ground line 42, which crosses under the storage capacitor xe2x80x9cSxe2x80x9d (see FIG. 2). A portion of the ohmic contact layer 91 on the active layer 86 is then etched to form a channel region using the source and drain electrodes 32 and 33 as masks. Thus, the TFT xe2x80x9cTxe2x80x9d (see FIG. 2) is complete.
Next, the first capacitor electrode 58 and the etch stopper 59 are respectively formed over the ground line 42 and over the gate pads 87 by depositing and patterning a transparent conductive material such as Indium-Tin-Oxide (ITO). The first capacitor electrode 58 is in electrical contact with the ground line 42. The etch stopper 59, as shown in FIG. 2, is arranged in a longitudinal direction while overlapping the gate pads 87. A dielectric protection layer 81 is then formed over the TFT, over the first capacitor electrode 58, over the etch stopper 59, and over the first insulation layer 75 by depositing Silicon Nitride (SiNx). Thus, the first insulation layer 75 and the protection layer 81 are stacked over the gate pads 87. The protection layer 81 also protects the TFT.
A second capacitor electrode 60, which corresponds in size to the first capacitor electrode 58, is then formed on the protection layer 81 and over the first capacitor electrode 58. The second capacitor electrode 60 is beneficially comprised of transparent conductive material such as Indium-Tin-Oxide (ITO) or Indium-Zinc-Oxide (IZO). A second insulation layer 83 is then formed, beneficially by depositing an organic substance such as BCB (Benzocyclobutene). BCB is a good choice because it has a low dielectric permittivity.
FIG. 3D shows a step of forming contact holes. The second insulation layer 83 and the protection layer 81 are etched to form a drain contact hole 85 over the drain electrode 33. Simultaneously, a capacitor electrode contact hole 95 is formed by etching the second insulation layer 83 over the second capacitor electrode 60. Also simultaneously, by etching the second insulation layer 83 and the protection layer 81, an etch stopper contact hole 97 over the etch stopper 59 is formed.
In order to form each contact hole, the dry-etching method is used. When performing the dry-etching method the dry-etch generates static electricity. The etch stopper 59 and the gate pads 87 are especially easy to charge with the static electricity. Charged gate pads can cause gate line defects. For example, electric charge on the gate pads 87 are readily conveyed to the gate lines, wherein they can accumulate until a static discharge occurs, destroying a gate line and causing an open circuit.
Referring now to FIG. 3E, a pixel electrode 62, which connects to the drain electrode 33 via the drain contact hole 85, and to the second capacitor electrode 60 via the capacitor electrode contact hole 95, is formed by depositing and patterning a transparent conductive material such as ITO (indium-tin-oxide) or IZO (indium-zinc-oxide). Since the pixel electrode 62 is conductive, the pixel electrode 62 and the second capacitor electrode 60 have equipotentials. When forming the pixel electrode 62, the etch stopper 59 (see FIG. 3D) over the gate pads 87 is etched. Then, gate pad contact holes 96 over the gate pads 87 are formed by etching the first insulation layer 75.
With respect to the above-mentioned processes, the reason for forming the etch stopper is to control the etch ratio when forming the contact holes. A more detailed explanation is now provided. When dry etching, the etching process is controlled by monitoring a gas that is produced by a chemical reaction between the etching gas and the insulation or protection layers using an electrical device, referred to as an EPD (end point detector). The EPD converts the amount of the produced gas to an electrical voltage. Thus, the duration of the etching can be controlled based upon the electrical voltage. However, it is difficult to accurately control the etching process. Referring now to FIGS. 3D and 3E, as noted, the drain contact hole 85 is formed by etching the second insulation layer 83 and the protection layer 81, and the capacitor electrode contact hole 95 is formed by etching the second insulation layer 83. Additionally, if the etching stopper 59 was not formed on the first insulation layer 75, the gate pad contact holes 96 would be formed by etching the second insulation layer 83, the protection layer 81 and the first insulation layer 75. Furthermore, all those openings would be etched at the same time. Thus, the drain electrode 33 and the second capacitor electrode 60 would likely be over-etched while forming the data pad contact holes 96. Therefore, the etch stopper 59 is formed on the first insulation layer 75 and over the gate pads 87 to enable more controlled etching.
While generally successful, some problems occur when practicing the above-mentioned process. For example, during fabrication, a significant amount of electric charge is stored during dry etching in a capacitor comprised of the first insulation layer 75, the etch stopper 59, and the gate pads 87. As previously explained, the stored charge can damage the gate lines, reducing the throughput and yield of the X-ray detector.
This invention has been developed in order to address the above-described problem.
An object of this invention is to provide an array substrate for use in an X-ray sensing device. Furthermore, it is an object of the present invention to reduce open gate lines caused by static electricity.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from that description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In order to accomplish at least one of the above objects, the principles of the present invention provide a method of fabricating an array substrate for use in an X-ray sensing device. The method includes forming a plurality of gate lines on a substrate, with the gate lines each having a gate electrode, a gate line extension and a gate pad. Then forming a first insulation layer on the gate line, on the gate electrode, on the gate line extension, on the gate pad and on the substrate. Next, forming first and second gate line contact holes to the gate line extension by patterning the first insulation layer. The method continues by forming a semiconductor layer on the first insulation layer and over the gate electrode, with the semiconductor layer including an active layer and an ohmic contact layer. Next, forming source and drain electrodes, a data line and a ground line, with the source and drain electrodes extend over the active layer, with the data line electrically connected to the source electrode, with the gate line and the data line defining a pixel region, and with the ground line crossing the pixel region. Next, forming a thin film transistor (TFT) near the crossing of the gate and data lines, with the TFT being comprised of the gate electrodes, the data electrode, the drain electrode and the semiconductor layer. Next, forming a first capacitor electrode, a plurality of island-shaped transparent electrode patterns, and an etch stopper, with the first capacitor electrode electrically contacting the ground line, with the electrode patterns formed over the gate line extensions and electrically contacting a pair of gate extensions via the first and second gate line contact holes, and with the etch stopper located over the plural gate pads. Then, forming a protection layer on the thin film transistor, on the first capacitor electrode, on the island-shaped transparent electrode patterns and on the etch stopper. Next, forming a second capacitor electrode on the protection layer and over the first capacitor electrode. Next, forming a storage capacitor in the pixel region, with the storage capacitor being comprised of the first capacitor electrode, the second capacitor electrode, and the protection layer. The method continues by forming a second insulation layer on the protection layer and on the second capacitor electrode. Then, forming an etching hole and an etch stopper contact hole, which respectively expose the island-shaped transparent electrode pattern and the etch stopper, by etching the second insulation layer and the protection layer. Then, etching the etch stopper and the portion of the island-shaped transparent electrode pattern.
A method of fabricating an array substrate further includes the step of forming a drain contact hole by etching the second insulation layer and the protection layer to expose the drain electrode. The method also includes the step of forming a capacitor electrode contact hole by etching the second insulation layer to expose the second capacitor electrode.
The drain contact hole, the capacitor electrode contact hole, the etching hole, and the etch stopper contact hole are beneficially formed in the same etching step.
A method of fabricating an array substrate further includes depositing a conductive material on the second insulation layer, in the drain contact hole, and in the capacitor electrode contact hole, and then patterning the conductive material to form a pixel electrode that electrically connects to the drain electrode and to the second capacitor electrode.
Beneficially, the first insulation layer is made of a material selected from a group consisting of Silicon Nitride (SiNx), Silicon Oxide (SiOx), BCB (Benzocyclobutene) and an acryl. The first capacitor electrode, the second capacitor electrodes, and the pixel electrode are made of a transparent conductive material such as Indium-Tin-Oxide (ITO) or Indium-Zinc-Oxide (IZO).
The protection layer is beneficially of Silicon Nitride (SiNx) and the second insulation layer is beneficially of BCB (Benzocyclobutene). The gate line extension beneficially extends from the gate line in a longitudinal direction, and the width of the gate line extension is wider than that of the gate line.
In order to accomplish the above objects, the principles of the present invention further provide for an array substrate. That array substrate includes a plurality of gate lines on a substrate, with each gate line having at least one gate electrode, a gate line extension and at least one gate pad. The array substrate further includes a first insulation layer is over the gate lines, over the gate electrodes, over the gate line extensions, and over the substrate. Additionally, a plurality of gate line contact holes pass through the first insulation to the gate line extensions. A plurality of thin film transistors are on the first insulation layer, each thin film transistor being located over a gate electrode and having a source electrode, a drain electrode, and a semiconductor layer. A plurality of data lines are also on the first insulation layer, each data line electrically contacting to a plurality of source electrodes and crossing over a plurality of gate lines to define a plurality of pixel regions. A ground line extends under each pixel region, with the ground line being on the first insulation layer. A first capacitor electrode is disposed over the ground line and over a portion of the first insulation layer. The first capacitor electrode electrically connects to the ground line. The first capacitor electrode is comprised of a transparent conductive material. A plurality of island-shaped transparent electrode patterns are disposed over the gate line extensions, with each island-shaped transparent electrode pattern contacting a pair of the gate line extensions via the first and second gate line contact holes. An etch stopper is disposed on the first insulation layer over the gate pads. A protection layer is disposed over the thin film transistor, over the first capacitor electrode, over the island-shaped transparent electrode pattern, and over the etch stopper. A second capacitor electrode is disposed on the protection layer and over the first capacitor electrodes. A second insulating layer is disposed over the protection layer and over the second capacitor electrodes. A plurality of etching holes pass through the second insulation layer and through the protection layer to the island-shaped transparent electrode pattern. Additionally, etch stopper contact holes pass through the second insulation layer and through the protection layer to the etch stopper.
An array substrate in accord with the principles of the present invention can further include a capacitor electrode contact hole through the second insulation layer that extends to the second capacitor electrode. Additionally, a drain electrode contact hole through the second insulation layer and through the protection layer extends to a first drain electrode of the plurality of drain electrodes.
An array substrate in accord with the principles of the present invention can further includes a pixel electrode that electrically contacts the drain electrode through the drain electrode contact hole.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.